Where did he get all his info on the process of creating the original Death Knights? Access to this info would have probably been insanely difficult to acquire for a member of the Shadow Council as Gul'Dan would have had little time to record his success before his demise, should he have even done so. I'm not saying it's impossible, but you need to have a really plausible way for this to happen. Perhaps your warlock was a membber of the Violet Eye and found something in the newly opened Karazhan's library that allowed you to replicate the process.

Why would he want to do this process on himself? Is his body weak? Old? Sheer desire for a stronger body? Did he test it first? Usually you don't go doing stuff like this without testing. These reasons could relate back to his life pre-warlock. Did he have an early life that made him feel powerless? Was his life great until something made him feel powerless? Is he just a person who likes to have power over others for no real particular reason? Before he was a warlock he was likely a mundane person with a mundane life, how did that change?

Did you really replicate the process exactly? Perhaps there were some small differences. There could be a flaw, there could be simply slight differences that may reflect your own background or the likely second-hand source's own misinterpretations. It may also relate to the fact that the corpse you are inhabiting is already going to be warped and full of necromantic energies. All necromantic energies are not created equal either as these helped the Lich King mantain control over said Scourgelord.

It would be nice to have more info on why and how he chose this Scourgelord. You gave a little reasoning, but it would be great to have backstory as to how he got the opportunity to accompany the assault force that made this target viable. Perhaps this chance also allowed for some inferior, but acceptable options even if the Scourgelord's corpse wasn't available. Perhaps you convinced some of the KotEB members that you wanted personal vengeance, and instead of stealing the corpse they handed it over freely.

Consider how you are going to establish a new identity. That saronite Scourgelord armor is nice, but you probably need to pass yourself off as a KotEB or alter the armor just enough to make sure people don't mistake you for a member of the Scourge. Even outside the cities it is not safe and the KotEB and AC will hunt you as long as they think you are an enemy. You will also probably have to cover your face, or find a way to alter it. Although the game lets us wear the armor of the enemy, it's not realistically a good idea, not unless it is altered in a manner to make it appear stolen. Tabard choice may be a good way to display this in game. This is all assuming you want to not be mistaken for said Scourgelord.

Last Question.... Why? All of this background is nice, but what's the purpose of RPing this character? Is it plausible? Yeah, but it's way over the top UNLESS you have an RP group into which this will work. That or of course if you're just trying to make a heroic background for the fun of it. RPing it could also be interesting if you play it in a subtle manner.

Anyway you need not answer any of these questions on here (unless you want more feedback). Hope it helps.